It is highly desirable to reduce the cycle time needed to cure a rubber article since more articles can be cured in a given mold per unit time. Reducing the duration of a cure cycle also generally results in energy savings.
Organic accelerators have been used for many years in order to make rubbers cure more quickly. Aniline was one of the first organic compounds used to accelerate the vulcanization of rubber. However, it was objectionable because of its toxicity. It was later discovered that the reaction product of aniline with carbon disulfide, namely thiocarbanilide, was more effective as an accelerator and less toxic. It then became the first widely used organic accelerator. In 1921, mercaptobenzothiazole was introduced as an accelerator for rubber vulcanization and it is still being used commercially as an accelerator today. Currently, a wide variety of accelerators are being utilized commercially including, thiazole accelerators such as benzothiazyl disulfide and 2-mercaptobenzothiazole: sulfenamide accelerators, such as N-oxydiethylene benzothiazole-2-sulfenamide and N-cyclohexyl-2-benzothiazolesulfenamide; dithiocarbamate accelerators, such as, bismuth dimethyldithiocarbamate, cadmium diethyldithiocarbamate, copper dimethyldithiocarbamate, lead dimethyldithiocarbamate, selenium diethyldithiocarbamate, and zinc dimethyldithiocarbamate; thiuram accelerators, such as, tetramethylthiuram disulfide and tetraethylthiuram disulfide: and thiourea accelerators, such as, tetramethylthiuram monosulfide, dimethylethyl thiourea, and trimethyl thiourea. Commercial or organic accelerators of this type can reduce the time required to vulcanize a rubber by at least 50 percent.
The incorporation of an accelerator into a rubber generally results in the rubber having a greater tendency to scorch. Scorch is the premature crosslinking of an elastomer which can render the elastomer totally unsuitable for many applications. Improving a rubber's cure rate without adversely affecting its scorch safety or other properties has proven to be a very formidable problem.